Rigidity is the most important functional quality of cardboard. The rigidity is particularly important to cardboard used for packages to make them easy to grip. The rigidity is expressed by the bending resistance or by the bending resistance index of the cardboard. The rigidity of cardboard is primarily adjusted by means of its grammage, higher grammage resulting in greater rigidity. In the case of identical grammage, the rigidity of the cardboard is determined by several parameters, principally its thickness and its tensile stiffness index, which is tensile stiffness/grammage. Great thickness is very important to rigid cardboard. A common value of the rigidity of cardboard for liquid cartons is a bending resistance index of about 20 Nm.sup.6 /kg.sup.3 or lower.
The grammage, which according to that stated above is related to the rigidity, is another important quality of cardboard. A low grammage implies little consumption of material, and therefore this is desirable from an economic point of view. A common value of the grammage of cardboard for 2-litre beverage cartons is about 360 g/m.sup.2.
In addition to rigidity and grammage, cardboard should in most cases satisfy other requirements as well. Thus, many converting operations require a certain degree of z-strength of the cardboard. The convertibility implies that the cardboard can be creased, punched and, for instance, covered with a plastic coating. In this connection, the z-strength is important to avoid undesired delamination of the cardboard, for instance when covering it with a plastic coating. if the cardboard has too little z-strength, the cooling roll adheres to the plastic during the extrusion coating and the cardboard is delaminated when being removed from the cooling roll. A satisfactory degree of z-strength is also necessary to obtain a good runnability, e.g. when splicing rolls, so-called flying splice. For a cardboard to be considered to satisfy the requirements of convertibility, it should have a z-strength of at least about 100 J/m.sup.2, and preferably of at least about 120 J/m.sup.2. A common value for cardboard is about 180 J/m.sup.2 or higher. The z-strength is usually adjusted by beating, increased beating resulting in a higher degree of fibre bonding and greater z-strength. However, at the same time the density increases and both thickness and rigidity decrease.
In addition to rigidity, printability is important. Qualities conditioning printability are brightness, measured as ISO brightness, whiteness and roughness, measured e.g. as a Bendtsen roughness number. In general, the ISO brightness should be at least about 72%, and the Bendtsen roughness should be about 800 ml/min at the most. A greater roughness of the cardboard can be tolerated if print is applied on the plastic coating or if the cardboard is laminated with a preprinted plastic film.
It is apparent from that stated above that several qualities as mentioned are in opposition to each other, an improvement of one quality at the same time leading to a deterioration of another quality. Thus, it is understood that it is very difficult to simultaneously obtain in cardboard the maximal values strived for of all the qualities mentioned above, in particular the values of rigidity, grammage and z-strength.
The following references are examples of prior-art techniques within this field.
From the Canadian patent specification 1,251,718, a multi-layer paper board is known, in which the central layer has a high density of about 550-770 kg/m.sup.3. The central layer consists of 30-70% chemically modified thermomechanical pulp (CTMP) and the rest of long fibre sulphate pulp to obtain a sufficient z-strength.
From U.S. Pat. No. 5,244,541, it is known to increase the z-strength and decrease the density by treating fibres of mechanical pulp mechanically and bending them. This additional mechanical treatment demands, however, a great deal of energy, and moreover bent fibres normally yield a more flocculant sheet, that is an inferior formation. Further, bent fibres deteriorate the tensile stiffness as well as the compression strength.
In WO 95/26441, a multi-layer cardboard material is described, in which the core has a high bulk, that is a low density. This is achieved by the use of cellulose fibres, which are cross-linked with the aid of synthetic binding agents, such as modified starch, polyvinyl alcohol, polyacrylates, different acrylate copolymers, etc.
U.S. Pat. No. 5,147,505 discloses a multi-layer paper, in which coarse fibres are used for the outer plies and finer fibres are used for the core. According to this patent, the finer fibres of the core influence the paper rendering it a good smoothness.
From DE 2,360,295 a high absorption offset-cardboard is known, whose core consists of groundwood pulp and cold water soluble starch.
U.S. Pat. No. 4,913,773 describes a multi-ply paperboard, which is distinguished by great rigidity in relation to grammage. This is achieved by using special fibres in the core that are kinked and curled.